The invention relates to a clutch mechanism with at least two contacting clutch surfaces, that is, a driving and a driven surface, with a meltable, fusible material interposed between either clutch lining face and its supporting structure to allow slippage in the event of an overtemperature condition. Present dry and wet type clutches include two engaging surfaces, a driven and a driving surface. In a typical clutch application, such as in an automobile drive train, the clutch consists of three basic elements. The driving face is attached or connected to the engine crankshaft, and revolves with it. A driven face is attached to a cover or housing which is bolted to the flywheel. The disc (the third clutch element) is squeezed between the driven and the driving surfaces during engagement of the clutch. At clutch engagement a certain amount of heat is generated. The rate of heat generation during engagement varies with the torque capacity of the clutch and the length of engagement time.
Dry clutches, that is, clutches not operating in a fluid environment, are not particularly susceptible to damage from the heat generation rate as they incorporate large heat sinks in the masses of their pressure plate and the engine flywheel. However, a dry clutch is susceptible to damage from the total amount of heat generated. All the heat in a dry clutch must be transferred to the surrounding air; therefore the time from engagement to lockup should be as short as possible. Wet clutches, that is, clutches cooled by oil or some other fluid, are more susceptible to damage from a high rate of heat generation, rather than from seeing a lower heat level for a long period of time during a clutch engagement cycle.
Heat affects every factor influencing a clutch assembly and its concomitant torque capacity. Heat causes mating surface warpage and dishing, and results in a dimensional shift of the effective mean radius of the clutch. The dishing can also cause a much higher rate of friction material wear as the mating surfaces will no longer make full contact, and torque capacity of the clutch will be reduced as the plate contact point moves and affects the effective mean radius. The coefficient of friction of the clutch facing material can change due to excessive heat and thereby cause the friction surface to break down and wear rapidly. Heat affects the effective number of active friction surfaces by causing warpage, uneven contact of the mating surfaces and generation of hot spots and premature failure of some plates. In addition, heat tends to remove the temper from springs thereby decreasing clamping force. Thus it is desirable to reduce the potential danger from heat generated during clutch engagement or from excessive slippage. One approach is to reduce clutch engagement time, but this adds to the shock load and potential damage hazard to the clutch and associated parts of the drive train, as well as being a discomfort to the operator.
It is apparent that clutch temperature is related to and generally increases with the energy dissipated by the clutch. It is known that during clutch operation the highest temperature is attained at the end of a range shift. Various devices have protected the connecting mechanical means of a drive system from damage due to excessive heat, torque or shearing forces. Examples of such devices are shear pins, meltable material requiring a replacement after use, a fracturable fusible material, or a combination of a fusible material with a mechanical shear pin. However, it is readily apparent that a shear pin is a unifunctional element, that is, it requires replacement after having performed its protective function. This can lead to the problem of replacement of a pin of the requisite size and material, as both characteristics can influence the load bearing potential of a shear pin. In the case of a meltable connecting material which is heat responsive and flows away from the mechanism, the same or similar problem obtains, that is, replacement of the material of the same composition and of the proper quantities required at the precise location within the mechanism.
It is thus an important object of the invention to provide a protective coupling device or mechanism that is heat responsive, and which does not require replacement after each use. Any protective device or mechanism for a clutch lining should operate in the range of about 350.degree. F. to 500.degree. F. to prevent slippage below 350.degree. but to protect from irreversible heat damage above about 500.degree. F.
A related object of the invention is to provide such a protective device or mechanism which allows protection of the engaging surfaces by affording controlled slip at excessive temperatures.